FIGS. 15(a)-15(c) show a prior art solar cell 51 with gallium arsenide (GaAs) laminated on a Si substrate as an example of the photovoltaic semiconductor device disclosed in Japanese Patent Publication 64-61958. FIG. 15(a) is a plan view showing a solar cell 51 with GaAs laminated on a Si substrate and FIGS. 15(b) and 15(c) are cross-sectional views along lines XVb--XVb and XVc--XVc of FIG. 15(a), respectively. An n-type GaAS layer 2, a p-type GaAs layer 3, and an anti-reflection film 5 are sequentially disposed on an n-type Si substrate 1. A bus electrode 6b of p-side electrode 6 is disposed on the Si substrate 1 with an intervening insulating film 8. The insulating film 8 prevents the n-side electrode 12 disposed on the rear surface of the Si substrate 1 from short-circuiting with the p-side electrode 6. A grid electrode 6a of p-side electrode 6 disposed on the p-type GaAs layer 3 is connected with the bus electrode 6b and part of the grid electrode 6a is disposed on the Si substrate 1, as shown in FIG. 15(c). The pn junction 13 of the GaAs solar cell layer and the grid electrode 6a are electrically separated by the insulating film 8.
In the solar cell 51 with GaAs layers 2 and 3 laminated on the Si substrate 1, the bus electrode 6b is disposed on the Si substrate 1 with the intervening insulating film 8. Therefore, stress arising during the welding of the external connector 9 is not directly applied to the GaAs layers and there is no cracking in the GaAs layers.
In the prior art solar cell 51 with GaAs laminated on an Si substrate 1, no problems occur when the external connector 9 is welded onto the p-side electrode 6. However, when a connector is welded onto the n-side electrode 12 disposed on the rear surface of the Si substrate 1, cracks occur in the Si substrate 1 and the GaAs solar cell layers 2 and 3, collectively referred to as layer 24 and so designated in FIGS. 16 and 17.
Usually, GaAs layers 2 and 3 are epitaxially grown on the Si substrate 1 at a high temperature, about 700.degree. C., and then the substrate is gradually cooled. In this process, the stress due to the difference in the lattice constants of Si and GaAs at the interface of the Si substrate 1 and the GaAs solar cell layers 2 and 3 is relieved because the degree of freedom of the atoms is high at about 700.degree. C. However, when the substrate is cooled to about 350.degree. C., the degree of freedom of the atoms is reduced and the stress cannot be relieved. Accordingly, due to the different thermal expansion coefficients of Si (2.4.times.10.sup.-6 K.sup.-1) and GaAs (5.7.times.10.sup.-6 K.sup.-1), the solar cell layer 24 becomes convex and stress accumulates inside the GaAs layer. This stress is sufficient to crack the GaAs layers 2 and 3. Although it may be possible to resist this stress in a static state, when a dynamic stress, such as a mechanical or a thermal stress, is applied, it is quite likely that cracking will occur in the GaAs solar cell layers 2 and 3 on the Si substrate 1.
A plurality of solar cells are usually connected by external connectors in a practical use. FIG. 17 shows a plurality of solar cells 51 connected by external connectors 9. The prior art solar cell 51 has an n-side electrode 12 on the rear surface of the Si substrate 1. Therefore, when the external connector 9 is connected to the n-side electrode 12, the solar cell 51, which is convex, is placed on a base 14 and heated while pressure is locally applied to the solar cell 51 by the welding head 15, as shown in FIG. 16. A bending moment is applied to the whole of the solar cell 51 and a local thermal stress is further applied from the welding head 15. Therefore, cracking occurs in the GaAs layer 24, i.e., layers 2 and 3, where the welding head 15 contacts the solar cell and the GaAs solar cell layer 24 may be broken. Thereby, the photovoltaic characteristics of the solar cell layer 51 are adversely affected. In some cases, the Si substrate 1 peels off and the solar cell cannot be used.